Filmed over six years across three continents, *Mariana x BHP* follows the fight for justice after Brazil's largest environmental disaster: the collapse of the Fundão dam in 2015, operated by the mining companies Vale (Brazil) and BHP (Australia). Through the intertwined paths of Marcelo Krenak, an Indigenous leader; Mônica dos Santos, a survivor turned activist; and Tom Goodhead, a Welsh lawyer determined to challenge corporate power, the documentary follows how an unlikely friendship transforms into the world's largest collective environmental lawsuit, filed in the London courts on behalf of 620,000 affected people and for US$ 48 billion against the most powerful mining company on the planet, exposing the human face, the paradoxes, and the hidden forces behind a global struggle between justice and power. Ten years after the disaster, following an unprecedented judicial process, the English court holds BHP responsible for the Mariana tragedy, setting a historic precedent for other communities to demand justice beyond their borders.